1. Field of the Invention
This invention relates to a Double Pass Pump Raman oscillator in combination with an optical system for preventing pump energy not absorbed in the oscillator from reentering the pump source optical path.
The invention further relates to optimal conversion of all of the pump energy to a desired output wavelength of radiant energy by coupling the output of a Raman oscillator and the non-absorbed pump energy to a Raman amplifier.
2. Description of the Prior Art
Longitudinally pumped oscillators fall into two distinct categories; (1) a single pass pump (SPP) configuration in which the pump beam makes a single pass through the gain medium, and (2) a double pass pump (DPP) configuration in which the pump beam is totally reflected by one of the resonator mirrors of the oscillator to make a double pass through the gain medium. A type of laser oscillator of this type is comprised of a resonate optical cavity containing a gain medium positioned therein. A housing is provided as part of the oscillator to maintain the Raman active gas at a predetermined pressure.
Efficient Raman generation in gases is achieved through the use of a Raman oscillator which is pumped longitudinally by a semi-collimated beam of an exciting laser. In the visible and near infrared portion of the spectrum, energy conversion efficiencies are typically on the order of 30 to 50%. A major problem associated with the implementation of a double pass pump oscillator concerns unused pump energy which exits back along the optical path of the pump source after a double pass in the resonate cavity. If this energy is permitted to be redirected back along the optical path of the pump source, serious amplitude stability and/or optical component damage could result. In addition, in some applications it is desirable to recover the unused pump radiation for use elsewhere in the optical system.
The energy conversion efficiency of the Raman oscillator is limited by the occurrence of parasitic oscillations and unwanted Stokes and antistokes orders. While careful attention to resonator mirror reflectivities may increase the maximum conversion efficiency attainable at the desired Stokes order, practical devices are limited to 40 to 50% conversion efficiencies from the original pump energy. Theoretical conversion efficiencies of the pump energy to the first Stokes order typically fall in the range from 70 to 90%. The theoretical energy conversion efficiency is based upon a photon in the pump beam being converted to a photon in the first Stokes order with the difference in photon energy being absorbed as heat in the gas in the form of molecular vibration or rotation.
Energy conversion efficiencies approaching the theoretical limit can be achieved by combining a Raman oscillator with a serial single pass Raman amplifier. The Raman oscillator is operated below the threshold of parasitic oscillations to provide an output of the first Stokes order plus the unused or unabsorbed pump energy. The unused pump energy is fed into the Raman amplifier along with the output of the Raman oscillator to further convert the unused pump energy to radiant energy of the wavelength of the Raman oscillator output. The gain level of the Raman amplifier is maintained below the avalanche threshold by delaying the unused pump energy exiting the Raman oscillator relative to the first Stokes radiation from the oscillator. Essentially 100% of the pump radiation injected into the Raman amplifier is converted into signal radiation of the first Stokes order.
In present systems, a Raman oscillator is provided by housing and active Raman gas at a pressure such as 1,000 psi in a vessel having windows to permit an optical path through the Raman active gas. Optical components are placed outside of the vessel in the optical path going through the vessel to permit a resonate cavity to be formed. In present systems, a separate Raman amplifier is provided housing an active Raman gas at a pressure of 1,000 psi. Windows are provided in the vessel to provide an optical path through the Raman active gas.
It is therefore desirable to provide optical components for efficiently recovering pump radiation exiting a double pass pump Raman oscillator where the pump radiation is either polarized or unpolarized.
It is further desirable to provide high energy conversion efficiency of the pump radiation by employing a common pressure vessel containing a Raman active gas for both the Raman oscillator and Raman amplifier.
It is further desirable to implement a single pass pump oscillator or a double pass pump oscillator in series with a Raman amplifier using a common pressure vessel holding Raman active gas with a plurality of optical channels or paths therethrough.